Optical fiber gratings are key components in modern telecommunication systems. Basically, optical fibers are thin strands of glass capable of transmitting an optical signal containing a large amount of information over long distances with very low loss. They are small diameter waveguides comprising a core having a first index of refraction surrounded by a cladding having a second (lower) index of refraction. Typical optical fibers are made of high purity silica with minor concentrations of dopants to control the index of refraction.
Optical fiber gratings are important elements for selectively controlling specific wavelengths of light within optical fibers. Optical fiber gratings include Bragg gratings, long period gratings and diffraction gratings. Such gratings typically comprise a body of material and a plurality of substantially equally spaced optical grating elements such as index perturbations, slits or grooves.
A typical Bragg grating comprises a length of optical fiber, including a plurality of index perturbations substantially equally spaced along the length. These perturbations selectively reflect light of wavelength .lambda. equal to twice the spacing .LAMBDA. between successive perturbations times the effective refractive index neff, i.e. .lambda.=2n.sub.eff .LAMBDA., where .lambda. is the vacuum wavelength. The remaining wavelengths pass essentially unimpeded. Such Bragg gratings have found use in a variety of applications including filtering, adding and dropping signal channels, stabilization of lasers, reflection of fiber amplifier pump energy, and compensation for waveguide dispersion.
Waveguide Bragg gratings are conveniently fabricated by doping a waveguide core with one or more dopants sensitive to ultraviolet light, e.g., germanium or phosphorous, and exposing the waveguide at spatially periodic intervals to a high intensity ultraviolet light source, such as an excimer laser. The ultraviolet light interacts with the photosensitive dopant to produce long-term perturbations in the local index of refraction. The appropriate periodic spacing of perturbations can be obtained by use of a physical mask, a phase mask, or a pair of interfering beams.
A difficulty with conventional Bragg gratings is that they filter only a fixed wavelength. Each grating selectively reflects only light in a narrow bandwidth centered around .lambda.=2n.sub.eff .LAMBDA.. However in many applications, such as wavelength division multiplexing (WDM), it is desirable to have a reconfigurable grating whose wavelength response can be controllably altered.
One attempt to make a tunable Bragg grating uses a piezoelectric element to strain the grating. See Quetel et al., 1996 Technical Digest Series, Conf on Optical Fiber Communication, San Jose, Calif., Feb. 25-Mar. 1, 1996, Vol. 2, p. 120, Paper No. WF6. The difficulty with this approach is that the strain produced by piezoelectric actuation is relatively small, limiting the tuning range of the device. Moreover, it requires a continuous application of electrical power with relatively high voltage of approximately 100 volts.
The second kind of fiber grating is the long period fiber grating. Long-period gratings provide wavelength dependent loss and may be used for spectral shaping. A long-period grating couples optical power between two co-propagating modes with very low back reflections. It typically comprises a length of optical waveguide wherein a plurality of refractive index perturbations are spaced by a periodic distance .LAMBDA.' which is large compared to the wavelength .lambda. of the transmitted light. In contrast with Bragg gratings, long-period gratings use a periodic spacing .LAMBDA.' which is typically at least 10 times larger than the transmitted wavelength, i.e. .LAMBDA.'.gtoreq.10.lambda.. Typically .LAMBDA.' is in the range 15-1500 micrometers, and the width of a perturbation is in the range 1/5 .LAMBDA.' to 4/5.LAMBDA.'. In some applications, such as chirped gratings, the spacing .LAMBDA.' can vary along the length of the grating.
Long-period fiber grating devices selectively remove light at specific wavelengths by mode conversion. In contrast with Bragg gratings in which light is reflected and stays in the waveguide core, long-period gratings remove light without reflection, as by converting it from a guided mode to a non-guided mode. (A non-guided mode is a mode which is not confined to the core, but rather, is defined by the entire waveguide structure. Often, it is a cladding mode). The spacing .LAMBDA.' of the perturbations is chosen to shift transmitted light in the region of a selected peak wavelength .lambda..sub.p from a guided mode into a nonguided mode, thereby reducing in intensity a band of light centered about the peak wavelength .lambda..sub.p. Alternatively, the spacing .LAMBDA.' can be chosen to shift light from one guided mode to a second guided mode (typically a higher order mode), which is substantially stripped off the fiber to provide a wavelength dependent loss. Such devices are particularly useful for equalizing amplifier gain at different wavelengths.
A difficulty with conventional long-period gratings is that their ability to dynamically equalize amplifier gain is limited because they filter only a fixed wavelength. Each long-period grating with a given periodicity (.LAMBDA.') selectively filters light in a narrow bandwidth centered around the peak wavelength of coupling, .lambda..sub.p. This wavelength is determined by .lambda..sub.p =(n.sub.g -n.sub.ng).multidot..LAMBDA.', where n.sub.g and n.sub.ng are the effective indices of the core and the cladding modes, respectively. The value of n.sub.g is dependent on the core and cladding refractive index while n.sub.ng is dependent on core, cladding and air indices.
In the future, multi-wavelength communication systems will require reconfiguration and reallocation of wavelengths among the various nodes of a network depending on user requirements. This reconfiguration will impact upon the gain of the optical amplifier. As the number of channels passing through the amplifier changes, the amplifier will show deleterious peaks in its gain spectrum, requiring modification of the long-period grating used to flatten the amplifier. Modifying the long-period grating implies altering either the center wavelength of the transmission spectrum or the depth of the coupling.
Thus, there is a need for tunable long-period gratings whose transmission spectra can be controlled as a function of the number of channels and power levels transmitted through an amplifier. It is desirable to have tunable long-period gratings which, upon activation, can be made to dynamically filter other wavelengths (i.e., besides .lambda..sub.p). It is also desirable to be able to selectively filter a broad range of wavelengths. Further, tunable long period gratings can be useful for suppressing amplifier spontaneous emission (ASE), and can also be used as tunable loss elements for filtering out undesirable remanent signals from communication channel ADD/DROP operations.
Diffraction gratings typically comprise reflective surfaces containing a large number of parallel etched lines of substantially equal spacing. Light reflected from the grating at a given angle has different spectral content dependent on the spacing. The spacing in conventional diffraction gratings, and hence the spectral content, is generally fixed.
In view of the foregoing, it can be seen that there is a need for tunable optical gratings including Bragg gratings, long-period gratings and diffraction gratings whose wavelengths can be latchably reconfigured.